The present invention relates to a thin film electroluminescent device using an improved modulation technique for providing a gray scale display.
Traditional thin film electroluminescent displays (TFEL) are typically constructed of a laminar stack comprising a set of transparent front electrodes, which are typically made of indium tin oxide, formed on a transparent substrate (glass), and a transparent electroluminescent phosphor layer sandwiched between front and rear dielectric layers situated behind the front electrodes. Disposed behind the rear dielectric layer are rear electrodes orientated perpendicular to the front electrodes. To illuminate an entire display, each row electrode is sequentially scanned and selected column electrodes are simultaneously energized with voltage pulses to illuminate selected pixels in a row. All rows are scanned in turn until the entire display has been illuminated thereby writing a frame of video data. This is sometimes referred to as a frame time addressing.
For monochrome and color displays a gray scale is a desirable feature in order to display video and graphic images with better screen clarity and definition. Current techniques to achieve a gray scale for thin film electroluminescent displays can be broadly categorized as those calling for modulation of the amount of charge flow through the phosphor layer. The present modulation techniques may be further divided into two subcategories, namely, amplitude modulation and pulse width modulation. These techniques have been used with traditional electroluminescent displays to achieve a gray scale.
Amplitude modulation is the modulation of the magnitude of the voltage pulses imposed across the electroluminescent layer. Different voltage pulse magnitudes within the operating range of the electroluminescent layer, which is typically 160 volts to 250 volts, cause different pixel brightnesses. Within certain limits a higher voltage pulse causes a greater amount of light to be emitted than a lower voltage pulse. Pulse width modulation is a single voltage pulse of a selected time duration imposed across the electroluminescent layer during each frame time to control the amount of light emitted from the pixel, which increases with increased duration of the voltage pulse. Both of these techniques are readily applied to an entire display by applying a voltage pulse to a row electrode and using varied magnitudes or duration of voltage pulses applied to the column electrodes thereby creating a gray scale display in a row by row manner.
Both of these modulation techniques control the charge transported through the electroluminescent layer to achieve a gray scale display, but the resultant optical performance and accuracy obtainable is not sufficient for the high number of luminescence levels desired in a true high resolution gray scale display. The electroluminescent layer has a nonlinear voltage versus luminescence curve that makes it difficult to obtain a desired luminescence output from the electroluminescent layer with different applied voltage pulse levels. Even if the applied voltage pulse levels are modified in some manner to compensate for such nonlinearities, the voltage versus luminescence curve tends to shift from location to location within a display and also varies from display to display. Additionally, individual pixels within the display may exhibit a voltage coupling to other pixels, which changes the actual voltage at a particular pixel, thereby changing the luminescence of the selected pixel. Furthermore, the voltage coupling varies with the particular pattern of voltages supplied to the entire display at any particular moment. The voltage coupling and the nonlinear voltage versus luminescence curve are especially prominent at low and intermediate voltage levels. These problems make it difficult to design displays with a high gray scale which requires accurate luminescence levels.
The refresh rates obtainable with traditional thin film electroluminescent displays are limited by the time required to address and illuminate each row within the display in a sequential manner by providing a single voltage pulse to a row electrode and a voltage pulse to selected column electrodes. After an entire display is refreshed by addressing and illuminating each selected pixel in a row by row manner, the process is repeated. The illumination rate (the rate at which voltage pulses are applied across the electroluminescent layer of the entire display to illuminate each pixel) is limited by the time required to address each pixel in a row-by-row manner, because the illumination and addressing functions of the display are combined. A typical display is refreshed at 60 Hz. Thus, frequency modulation techniques are not easily adaptable to conventional drive techniques because if the refresh rate falls too low, flickering will result and higher frame rates are limited by the RC time constants of the display.
Gray scale has been available for plasma displays which have a memory characteristic that is not present in thin film electroluminescent displays. A high voltage pulse, typically of 200 volts, is imposed on a pixel to cause the pixel of the plasma display to emit light. Thereafter, a sustaining signal, typically of 180 volts, is imposed on the pixel to maintain the emission of light from the pixel. While the pixel is emitting light, the frequency of the sustaining signal may be chosen to cause the pixel to emit the desired luminescence. However, if the sustaining signal is imposed on the pixel without an immediately prior high voltage pulse, the pixel will not emit light. Thus, the plasma display relies upon the inherent memory of the plasma medium to make gray scale possible. Gray scale approaches used for plasma display are not readily adaptable to TFEL displays, because the electroluminescent layer of TFEL displays do not exhibit the requisite memory characteristics of plasma displays.
What is desirable then, is a modulation scheme for TFEL displays to achieve more accurate luminescence levels, whereby, the effects of the nonlinear voltage versus luminescence curve are minimized, the effects of the voltage coupling are reduced, and which does not sacrifice the panel's high illumination rates.